Post on 04-Jan-2022
International Journal of Engineering and Techniques - Volume 7 Issue 5 - September 2021
ISSN: 2395-1303 http://www.ijetjournal.org Page 29
A NOVEL TECHNIQUE FOR MULTI USER MULTIPLE ACCESS SPATIAL
MODULATION USING ADAPTIVE CODING AND MODULATION
Urvashi Tembhare1, Kamal Niwaria2
1(M.Tech Scholar Electronics and Communication Department, RKDF Institute of Sciences &Technology, and
SRK University, Bhopal
Email: tembhareurvi0611@gmail.com)
2 (Asst. Professor Electronics and Communication Department, RKDF Institute of Sciences &Technology, and SRK
University, Bhopal
Email: kamalniwaria@gmail.com)
Abstract
The need for high peak data rates with the corresponding need for significantly increased spectral efficiencies, and
the support for service specific quality of service (QoS) requirements are the key elements that drive the research in
the area of wireless communication access technologies Since space constellations and signal constellations are
orthogonal, the well-known digital signal modulation schemes can be used on top. The spatial multiplexing gain
comes from the simultaneous transmission of spatially encoded bits. Analytical and numerical performance of SM in
different channel conditions, including practical channel considerations, are studied and compared to existing
MIMO techniques in this thesis. Results show that SM achieves low BER (bit error ratio) with a tremendous
reduction in receiver complexity without sacrificing spectral efficiency.
Key words- MIMO, Spatial Modulation, BER, Bit Error Rate, Modulation Technique etc.
I. INTRODUCTION
The upsurge of wireless communication systems
such as vehicle-to-vehicle (V2V) communication [1] and
wireless high definition (HD) television have fuelled
research into MIMO technology. In recent years, MIMO
has been considered as one of the core techniques for
improving data throughput, link reliability and spectral
efficiency [2-4]. MIMO techniques can be divided into
two main categories, namely spatial diversity and spatial
multiplexing schemes. Spatial diversity techniques [5, 6]
improve link reliability by transmitting multiple redundant
copies of data to a receiver over independent channels.
Alamouti’s scheme [6] is a popular transmit diversity
technique, which uses a pair of transmit antennas to
achieve full transmit diversity. However, diversity gains
are traded off by low spectral efficiency, which remains
unchanged when compared to a single-input multiple-
output (SIMO) system [7].
Among the set of existing technologies, multiple-
input multiple-output (MIMO) with adaptive coding and
modulation is promising candidate for future wireless
systems. MIMO system boosts the spectral efficiency by
using multiple transmit-antennas to simultaneously
transmit data to the receiver. Adaptive modulation and
coding enables robust and spectrally-efficient transmission
over time-varying channels. The basic premise is to
estimate the channel at the receiver and feed this estimate
back to the transmitter, so that the transmission scheme
can be adapted relative to the channel characteristics. The
idea is to consider the transmit antenna array as a spatial
constellation diagram with the constellation points being
the actual antennas. Each spatial constellation point is then
mapped to a different bit sequence. At any time instant,
only one antenna transmits energy, and the actual
information resides in the actual physical location of the
transmitting antenna, or spatial constellation point. This, of
course, requires a new detection process at the receiver,
namely the antenna detection.
IV. RELATED WORK
Space shift keying (SSK) modulation in [28] can be
considered as a special instance of SM, since only the
transmit antenna indices carry information. The SSK
scheme eliminated the use of conventional modulation
techniques, which reduced receiver complexity as
compared to SM. Furthermore, this decrease in receiver
complexity was attained without sacrificing performance
gains. Similar to SM, SSK schemes work only for a
number of transmit antennas which are a power of two. In
cases where the transmit antenna constraint cannot be met,
the generalized SSK (GSSK) scheme [29] provides a
viable solution. In [29], GSSK used a combination of
antenna indices to transmit information, which may be
International Journal of Engineering and Techniques - Volume 7 Issue 5 - September 2021
ISSN: 2395-1303 http://www.ijetjournal.org Page 30
applied to any antenna configuration. However, the
flexibility of GSSK is traded off by reduced performance
as compared to SSK [29].
Fractional bit encoded spatial modulation (FBE-
SM) in [30], which is a more versatile SM scheme based
on the theory of modulus conversion. The FBE-SM
approach allowed the transmitter to function with an
arbitrary number of antennas. As a result, this scheme is
well suited to compact mobile devices, where space
constraints pose limits on the number of transmit antennas.
Numerical results have shown that FBE-SM offers
flexibility in design and the necessary degrees of freedom
for trading-off attainable performance and capacity [30].
Trellis coded spatial modulation (TCSM) in [31],
which applied trellis coded modulation (TCM) to the
antenna constellation points of SM. This increased the free
distance between antenna constellation points, thus leading
to improved performance over spatially correlated
channels. The TCSM scheme was analyses in [32], where
an analytical framework for the performance over
correlated fading channels was proposed.
Soft-output ML detection in [33], where a soft-
output ML detector for SM orthogonal frequency division
multiplexing (OFDM) systems was introduced and shown
to outperform the conventional hard decision based SM
detector.
SM with partial channel state information (CSI) at
the receiver in [34], where an SM detector with unknown
phase reference at the receiver was developed and
analyzed. Monte Carlo simulations were used as a tool to
verify the analytical frameworks and investigate the
performance of the proposed detector. Results indicated
that SM performance was severely degraded when phase
information was not available at the receiver. This
highlights the fact that accurate channel estimation is
required for the efficient operation of SM detectors [34].
Optical spatial modulation (OSM) in [35], which is
an indoor optical wireless communication technique based
on SM. The OSM scheme was shown to achieve twice and
four times the data rate as compared to conventional on-off
keying and pulse-position modulation techniques,
respectively [35]. In [36], channel coding was applied to
OSM and the BER performance of both hard and soft
detectors were determined analytically. Monte Carlo
simulation results demonstrated that the application of
channel coding techniques can enhance OSM performance
by approximately 5dB and 7dB for hard and soft decisions,
respectively [36].
Normalized maximum ratio combining (NMRC)
detector in [37], where a low complexity sub-optimal SM
detection algorithm for unconstrained channels was
proposed. In addition, an antenna index (AI) list based
detector was also introduced in [37]. Monte Carlo
simulation results and corresponding analysis indicated
that the AI list based scheme can achieve near optimal
performance and reduced complexity when compared to
the optimal SM detector. However, the AI list based
detector can only operate efficiently for list sizes equal to
half the number of transmit antennas. This resulted in a
40% increase in complexity as compared to the NMRC
scheme.
Space-time block coded spatial modulation (STBC-
SM) in [38], which combined SM and STBC in order to
exploit the transmit diversity potential of MIMO channels.
The proposed scheme was analyzed in [38], where a closed
form expression for the average BER was derived. Monte
Carlo simulations results were used to support analytical
frameworks and also demonstrated the performance
advantages of STBC-SM over SM. Results indicated that
STBC-SM offers performance enhancements of 3dB-5dB
(depending on the spectral efficiency) over conventional
SM [38]. It must be highlighted that a similar scheme
termed Alamouti coded spatial modulation is proposed in
this dissertation. However, both schemes have been
developed independently based on the different paradigms
employed.
SM–MIMO
In this section, we commence by introducing the
SM– MIMO concept, illustrating it with the aid of some
simple examples. Again, we denote by Nt and Nr the
number of TAs and RAs, respectively. The cardinality of
the signal constellation diagram is denoted by M. Either
PSK or QAM are considered. In general, Nt, Nr, andM can
be chosen independently of each other. At the receiver,
optimum ML demodulation is considered. Thus, Nr can be
chosen independently of Nt . For ease of exposition, we
assume 𝑁𝑡 = 2𝑛𝑡 and 𝑀 = 2𝑚with nt and m being two
positive integers. In Section IV, we describe general SM–
MIMO encodings as well as some suboptimal (non-ML)
demodulation schemes.
In Fig. 1, the SM–MIMO concept is illustrated for
Nt = M = 2, and it is compared to the conventional SMX
scheme and the OSTBC scheme designed for transmit
diversity. In the latter case, the Alamouti scheme is
considered as an example [80].
International Journal of Engineering and Techniques - Volume 7 Issue 5 - September 2021
ISSN: 2395-1303 http://www.ijetjournal.org Page 31
In SMX–MIMO, two PSK/QAM symbols (S1 and
S2) are simultaneously transmitted from a pair of TAs in a
single channel use. For arbitrary Nt and M, the rate of
SMX is 𝑅𝑆𝑀𝑋 = 𝑁𝑡𝑙𝑜𝑔2(𝑀)bpcu.
Figure 1 Illustration of three MIMO concepts: (a) spatial
multiplexing; (b) transmit diversity; and (c) SM.
In OSTBC–MIMO, two PSK/QAM symbols (S1
and S2) are first encoded and then simultaneously
transmitted from a pair of TAs in two channel uses. For
arbitrary Nt and M, the rate of OSTBC is 𝑅𝑂𝑆𝑇𝐵𝐶 =
𝑅𝑐𝑙𝑜𝑔2(𝑀)bpcu, where 𝑅𝑐 = 𝑁𝑀 𝑁𝑐𝑢⁄ ≤ 1 is the rate of
the space-time block code and NM is the number of
information symbols transmitted in Ncu channel uses If, as
shown in Fig. 1, the Alamouti code is chosen, then we
have Rc = 1.
In SM–MIMO, only one (S1) out of the two
symbols is explicitly transmitted, while the other symbol
(S2) is implicitly transmitted by determining the index of
the active TA in each channel use. In other words, in SM–
MIMO, the information symbols are modulated onto two
information carrying units: a) one PSK/QAM symbol; and
b) a single active TA via an information-driven antenna-
switching mechanism. For arbitrary Nt and M, the rate of
SM is𝑅𝑆𝑀 = 𝑙𝑜𝑔2(𝑀) + 𝑙𝑜𝑔2(𝑁𝑡) bpcu [76], [82].
In Figs. 2 and 3, the encoding mechanism of SM–
MIMO is illustrated for Nt = M= 4 by considering two
generic channel uses, where the concept of ‘‘SM or
spatialconstellation diagram’’ is also introduced. The rate
of this MIMO setup is 𝑅𝑆𝑀 = 𝑙𝑜𝑔2(𝑀) + 𝑙𝑜𝑔2(𝑁𝑡) = 4
bpcu, hence the encoder processes the information bits in
blocks of four bits each. In the first channel use shown in
Fig. 2, the block of bits to be encoded is ‘‘1100.’’ The first
𝑙𝑜𝑔2(𝑁𝑡) = 2bits, ‘‘11,’’ determine the single active TA
(TX3), while the second𝑙𝑜𝑔2(𝑀) = 2 bits, ‘‘00,’’
determine the transmitted PSK/QAM symbol. Likewise, in
the second channel use shown in Fig. 3, the block of bits to
be encoded is ‘‘0001.’’ The first 𝑙𝑜𝑔2(𝑁𝑡) = 2 bits, ‘‘00,’’
determine the single active TA (TX0), while the second
𝑙𝑜𝑔2(𝑀) = 2 bits, ‘‘01,’’ determine the transmitted PSK/
QAM symbol.
The activated TA may change every channel use
according to the input information bits. Thus, TA
switching is an effective way of mapping the information
bits to TA indices and of increasing the transmission rate.
It is worth mentioning here that the idea of increasing the
rate of wireless communications using TA switching has
been alluded in pioneering MIMO papers under the
concept of ‘‘spatial cycling using one transmitter at a
time’’.
Figure 2 Illustration of the 3-D encoding of SM (first channel
use).
Figure 3 Illustration of the 3-D encoding of SM (second channel
use).
The information bits are modulated onto a 3-D
constellation diagram, which generalizes the known 2-D
(complex) signal-constellation diagram of PSK/QAM
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ISSN: 2395-1303 http://www.ijetjournal.org Page 32
modulation schemes. The thirddimension is provided by
the antenna array, where some of the bits are mapped to
the TAs. In SM–MIMO research, this third dimension is
termed the ‘‘spatial-constellation diagram’’ [76]. In simple
mathematical terms, the signal model of SM– MIMO,
assuming a frequency-flat channel model, is as follows:
𝐲 = 𝚮𝐱 + 𝐧
where 𝐲 ∈ ℂ𝑁𝑟×1 is the complex received vector;
𝚮 ∈ ℂ𝑁𝑟×𝑁𝑡 is the complex channel matrix; 𝐧 ∈ ℂ𝑁𝑟×1is
the complex AWGN at the receiver; and 𝐱 = 𝐞𝑠 ∈ ℂ𝑁𝑡×1
is the complex modulated vector with 𝑠 ∈ ℳ ⊆ ℂ1×1
being the complex (scalar) PSK/QAM modulated symbol
belonging to the signal-constellation diagram and 𝐞 ∈ 𝒜
being the Nt×1 vector belonging to the spatial-
constellation diagram𝒜 as follows:
𝐞𝑡 = {1, if the 𝑡th TA is active
0, if the 𝑡th TA is not active
where et is the tth entry of e for t = 1, 2,...,Nt. In
other words, the points (Nt-dimensional vectors) of the
spatialconstellation diagram are the Nt unit vectors of the
natural basis of the Nt-dimensional Euclidean space.
If Nt = 1, SM–MIMO reduces to conventional
singleantenna communications, where the information bits
are encoded only onto the signal-constellation diagram. In
this case, the rate is𝑅0 = 𝑙𝑜𝑔2(𝑀). On the other hand, if M
= 1 the information is encoded only onto the spatial-
constellationdiagrambyproviding etequalto𝑅𝑆𝑆𝐾 =
𝑙𝑜𝑔2(𝑁𝑡). In particular, SSK modulation is a MIMO
scheme, where data transmission takes place only through
the informationdriven TA switching mechanism. It is
apparent that SM– MIMO can be viewed as the
combination of single-antenna PSK/QAM and SSK–
MIMO modulations.
II. PROPOSED METHODOLOGY
The system model of the proposed 2 user MA-SM
with AMC is shown in Fig. 4, which consists of a MIMO
wireless link. We have two user transmitters, transmitting
over a Rayleigh fading channel to a common receiver. The
receiver is a SM demodulator with a ML detector
explained in previous section. The receiver calculates the
joint probability of the two received signals. The ML
detector then computes the Euclidean distance between the
received vector signal and the set of all possible received
signals, selecting the closest one.
Figure 4. 2-user Multiple Access Spatial Modulation with ACM
System Model
At the user end, in contrast to conventional SM
which uses the same modulation order for data mapping, in
the proposed scheme, the modulation orders assigned to
transmit antennas are chosen by the switching unit. More
specifically, when the channel is slowly varying, the
adaptive unit at the receiver computes the optimum
candidate for transmission and sends this information to
the two users through a low-bandwidth feedback path. The
transmitters then employ the corresponding modulation
orders for the next channel use.
The Adaptive unit uses the channel state
information obtained by the receiver to decide the
optimum level of modulation. It then relays it back to the
transmitters of the two users, which utilize this information
to modulate the next transmitted signal accordingly. If the
nature of the channel changes, then so does the modulation
order. This is done by predefining a minimum and
maximum level of BER. The receiver then compares the
BER of the received signal to the pre-set values, for a
signal that has a lower BER value than desired, the frame
size of the transmitted signal can be increased and for a
signal that has higher BER value than desired, the frame
size of the transmitted signal is reduced. Quintessentially,
we aim to optimally use the channel bandwidth available
to us. According to the requirement and characteristics of
the channel, the transmitter adapts itself to the best
modulation scheme so as to maintain BER for better
spectral efficiency.
V. RESULTS AND SIMULATION
In this section, we study and compare the graphs
obtained for various levels of channel attenuation with and
without ACM.
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ISSN: 2395-1303 http://www.ijetjournal.org Page 33
Each simulation graph shown below consists of
four results. The pink and cyan graphs depict the results
for two user multiple access spatial modulation without
ACM. The red and blue graphs denote the outcomes of our
proposed methodology with ACM. As can be seen in the
following section, we consider the different variations in
the simulation results by varying the channel attenuation of
the system. This change in channel attenuation leads to
change in the CSI and shows adaptive modulation in
action. The graphs study the BER of the system in
reference to the SNR in dB of the proposed scheme. The
main aim as stated is to minimize BER of the received
signal, and by merely studying the graph it is beautifully
apparent. Also, if we do a detailed deliberation of each
case and compare the results with previous outcomes, we
can easily see and analyze the changes in the various
outcomes for different inputs.
Figure 5 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 1 and 2 dB respectively.
This graph illustrates the behavior of the signal with
and without ACM for channel attenuation of User1 and
User2 as 1 and 2 respectively. For the graph without ACM
i.e. only multiple access spatial modulation the BER is
higher. It does gradually decrease with the SNR but
doesn’t go lower than 0.001 whereas in the presence of
ACM, the BER is more of a straight line and sharply
decreases to the lower bound values. The upper and lower
bound values of BER are predefined so as to maintain it in
a prescribed range and modulate accordingly.
Figure 6 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 2 and 1 dB respectively.
Now, this graph is a reversal of the previous one.
Here the values are similar except the fact that the channel
attenuation for user1 is now 2 and for user2 is 1. The
behaviour is identical as was in the previous graph. This is
because we are working in a system that calculates joint
probability of both the received signals and thus the result
always remains the same. The behavior is predominantly
dependant on the channel properties. This stems the fact
that irrespective of the number or position of the TA, the
behaviour of the receiver remains the same.
Figure 7 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 3 and 1 dB respectively.
In this graph, we increase the channel attenuation of
user1 to 3 while maintaining channel attenuation of user2
at 1. The behavior of user2 shows the BER is higher and
for user1 the BER is lower but more sharply decreasing. If
we compare it to the previous graph, where user2 was still
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ISSN: 2395-1303 http://www.ijetjournal.org Page 34
1 the graph is steeper and by increasing the channel
attenuation of user1, the receiver properties change to
lowering the overall value of BER.
Figure 8 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 0 and 1 dB respectively.
For a more in depth study, the channel attenuation
of user1 is further lowered to 0 and that of user2 is still 1.
The result now is vastly different as can be seen. For user1
transmission with and without ACM the BER of both is at
a constant value. This might be because the receiver can
no longer calculate the minimum distance and thus no
variation in the BER. Whereas for the case of user2, with
ACM the graph abruptly cuts off. This needs to be studied
in more depth to understand the behaviour of the systems.
Figure 9 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 10 and 1 dB respectively.
Let us up the ante now by taking channel
attenuation of user 1 as 10. When the difference in channel
attenuation value is high the receiver sends back
information to compensate for the BER and thus the BER
of the other user, user2 in this case drastically increases.
The BER of user1 is lower than in any of the previous
cases.
Figure 10 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 0.5 and 1 dB respectively.
This is an interesting graph. As we can see the
channel attenuation of user1 is 0.5 here i.e. between 0 and
1. The graph shows properties closer to the case of channel
attenuation being 1. This leads to the conclusion that if the
difference in channel attenuation is small then the
difference in the BER values also reduces. Although the
behaviour remains the same.
Figure 11 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 0.005 and 1 dB respectively.
Further decreasing the channel attenuation of user1
to 0.005 shows an entirely new result, which is somewhere
in between the BER values of users with channel
attenuation of 0 and 1. Both user1’s with and without
ACM, are nearly at a constant value as it was for the case
of channel attenuation 0. But the second users, just show a
more extended declining version the graph. It can be
assumed that the receiver is trying to balance the two
International Journal of Engineering and Techniques - Volume 7 Issue 5 - September 2021
ISSN: 2395-1303 http://www.ijetjournal.org Page 35
channel attenuation values for an efficient use of the
channel, which is actually what we set out to achieve.
Figure 12 Comparison Results for users 1 and 2 with and without
ACM and channel attenuation of 1 and 0.1 dB respectively.
For channel attenuation values of 1 and 0.1 for
user1 and user2 respectively. The graph for multiple
access SM case is not much different just a little lower
than it was before. This leads to the user with ACM to
overlap with it for the case of 0.1 channel attenuation.
III. CONCLUSION
The performance of 2 user multiple access spatial
modulation with adaptive coding and modulation was
studied. The ML receiver was successfully implemented in
the Rayleigh fading channel and used to enhance the
performance through ACM. For different channel
attenuation values and changing number of antennas the
impact on BER was studied. The BER shows a significant
improvement with ACM as opposed to without ACM. We
also studied the BER of the two users for various changes
in the channel attenuation. The receiver shows consistent
performance characteristics for both users, which was also
a point of concern. As we utilized the concept of joint
probability to optimize both of the obtained signals.
The generalization of this system for multiple user
with different modulation techniques can be studied in the
future. Applying coding schemes to this multiple user
interface should impact BER and can be further
investigated.In recent years, many solutions have been
proposed independently with the aim of simplifying the
design of MIMO communications. We believe that this
trend reinforces the potential impact of SM–MIMO
research in the context of next-generation wireless
systems.
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